Overall goals of this study are to evaluate the factors that alter the incidence of gallstone disease in African green monkeys (Cercopithecus althiops). For this we willtest the influence of a number of drugs and dietary factors on the composition of the bile as well as the formation of gallstones. Thirty-three animals divided into a 2 X 2 factorial design will be used to study the interaction of dietary fatty acids and oral contraceptives on the incidence of gallstone disease. We have shown previously that polyunsaturated fats increase the incidence of gallstones in both squirrel monkeys and African green monkeys. Others have reported an increased incidence of gallstones in women taking oral contraceptives. The purpose of this study is to determine whether there is an interaction of oral contraceptives with polyunsaturated fatty acids to produce an exacerbation in the incidence of gallstone disease. In a second experiment 7 African green monkeys will be studied for acute changes in bile lithogenicity following short-term administration of drugs that lower plasma cholesterol concentrations, such as clofibrate and cholestyramine. It is the purpose of this study to determine whether it is possible to evaluate the influence of various factors on the potential for forming gallstones by studying, acutely, the changes in bile composition. In addition to these studies, experiments will be conducted to determine the mechanism of changes in bile lithogenicity as well as to evaluate any changes in lipoprotein composition that might be associated with the increase or